Magnetic resonance imaging, further referred to as MRI, is of increasing relevance for modern diagnosis. Up to now active implanted medical devices (active IMDS), such as (but not restricted to) pacemakers, cardioverters/defibrillators, and cardiac resynchronization therapy devices, prevent effective utilization of MRI technology for diagnosis. Even the newly-presented so-called “MRI conditional” implants pose difficulties insofar as such devices have to be switched to a so-called “MRI safe” mode, which requires at least a consultation with a heart specialist before and after the MRI examination (if not requiring the presence of a heart specialist during the entire MRI examination).
These problems can be overcome by a reliable detection of MRI fields and appropriate programming of the active IMDs, as described in EP 1 935 450 by Digby et al. or by Shankar et al. in U.S. Pat. No. 6,963,779. Wahlstrand et al. disclose in U.S. Pat. No. 5,438,990 the use of a so called MagFET, a magnetic field effect transistor or magnetic field sensitive MOSFET as a reed switch. U.S. Pat. No. 5,179,429 to Ristic et al. discloses a magnetic field sensor based on a bipolar transistor with split collector contacts. Also known in the art is the use of hall sensors replacing the reed switch and enabling the device to detect the strength of a magnetic field, as in U.S. Pat. No. 6,510,345 Van Bentem et al.
These solutions still have power consumption which is unsuitable for use in a medical device. Therefore, it is an object of the invention to provide a sensor device accurate enough to discriminate between different magnitudes of a magnetic field, while at the same time consuming less power than prior solutions.